Printing apparatuses used as a printer, a copying machine, a facsimile apparatus, or an output apparatus for a multifunction electronic apparatus or work station including a computer or wordprocessor print images (including characters and the like) on printing media such as a printing sheet and thin plastic plate on the basis of image information (including character information and the like).
Such printing apparatuses can be classified by the printing method into an inkjet method, wire dot method, thermal method, electro-photographic method, and the like. Of printing apparatuses complying with these methods, a printing apparatus complying with the inkjet method (to be referred to as inkjet printing apparatuses hereinafter) prints by discharging ink from a printhead onto a printing medium. Compared to printing apparatuses according to other printing methods, the inkjet printing apparatus easily achieves high definition, high speed, quiet operation and low cost.
To meet recent needs for color printing, many color inkjet printing apparatuses have also been developed.
Generally in the inkjet printing apparatus, the integration of ink orifices and liquid channels serving as ink discharge portions is adopted in a printhead formed by integrating and arraying a plurality of printing elements. To cope with color printing, a plurality of printheads are mounted to the apparatus.
If an ink droplet discharged from the printhead is downsized to obtain a high-quality image almost free from graininess, density unevenness and color unevenness which have not occurred in a conventional printhead occur.
One of the factors that generate such density unevenness and color unevenness are probably the fact that the ink droplet adhering position on a printing medium in the main scanning direction periodically shifts by vibrations of a carriage due to a small ink droplet size in printing while the carriage to which the printhead is mounted moves in the carriage moving direction (main scanning direction). Also, density unevenness and color unevenness are probably attributed to the fact that the ink droplet adhering position on a printing medium in the printing medium conveyance direction (sub-scanning direction) periodically shifts in the conveyance direction (sub-scanning direction). Such shifts of the ink droplet adhering position in the main scanning direction and sub-scanning direction stand out more with a larger printing medium size and larger image data size.
In grayscale printing by an inkjet printing apparatus, a dot layout pattern corresponding to the gradation levels (also referred to as “quantization levels”) of pixels is assigned. The gradation level includes not only a halftone level of achromatic color but also a halftone level of chromatic color (e.g. cyan, magenta and yellow). For example, Japanese Patent Publication Laid-Open No. 9-46522 discloses a method of assigning plural types of dot layout patterns to a plurality of pixels at the same gradation level (quantization level). In this arrangement, dots are laid out at different intervals within a region formed by a plurality of pixels at the same gradation level, resulting in a noise-added printing state.
Even if the dot adhering position shifts along with the above-mentioned carriage movement or print medium conveyance operation, density unevenness is hardly recognized because noise is inherently added if plural types of dot layout patterns are used. However, if plural types of dot layout patterns are used, sparse and dense dot patterns are generated within a region formed by a plurality of pixels at the same gradation level. The sparse and dense dot patterns lead to graininess. Graininess becomes conspicuous especially at low gradation level.